Ha! This is a very standard question at the end of interviews. When I interview candidates I always ask it and try to pressure candidates to give me at least three questions, but when I was asked it I couldn't think of a thing. (I guess that's karma!) It wasn't nerves or apathy - but because TA is so thorough and communicative, by the time I got to this point in the process I honestly couldn't think of any additional questions. I would recommend preparing yourself a little list as you go as once you get excited talking about what they do and what you've done, it can be hard to recall any unfinished bases. You don't want to look like a goober like me ;P